The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While many varied packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted and interconnected on a small footprint substrate. The substrate may in general include a rigid, dielectric base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for connection of the die to a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound which provides a protective package.
A cross-sectional side view and a top view of a conventional semiconductor package 20 are shown in FIGS. 1 and 2 (without molding compound in FIG. 2). Typical packages include a plurality of semiconductor die, such as flash memory die 22 and controller die 24, affixed to a substrate 26. A plurality of die bond pads 28 may be formed on the semiconductor die 22, 24 during the die fabrication process. Similarly, a plurality of contact pads 30 may be formed on the substrate 26. Die 22 may be affixed to the substrate 26, and then die 24 may be mounted on die 22. All die may then be electrically coupled to the substrate by affixing wire bonds 32 between respective die bond pad 28 and contact pad 30 pairs. Once all electrical connections are made, the die and wire bonds may be encapsulated in a molding compound 34 to seal the package and protect the die and wire bonds.
In order to most efficiently use package footprint, it is known to stack semiconductor die on top of each other, either completely overlapping each other, or with an offset as shown in FIGS. 1 and 2. In an offset configuration, a die is stacked on top of another die so that the bond pads of the lower die are left exposed. An offset configuration provides an advantage of convenient access of the bond pads on each of the semiconductor die in the stack.
In order to increase memory capacity in semiconductor packages while maintaining or reducing the overall size of the package, the size of the memory die has become large compared to the overall size of the package. As such, it is common for the footprint of the memory die to be almost as large as the footprint of the substrate. Space within the semiconductor package for wirebonding down to the substrate is therefore at a premium. In particular, where there are multiple stacked flash memory die 22, it may become difficult to find space on the substrate for all of the contact pads required to make all of the necessary electrical connections. The number of die bond pads, contact pads and wire bonds in an actual semiconductor package would be many more than is shown in FIGS. 1 and 2. The number shown in FIGS. 1 and 2 is greatly reduced for the sake of clarity. Moreover, FIGS. 1 and 2 include only a pair of memory die 22. There may be more than that in the die stack, making it even harder to find room for all of the required wire bonds.
The controller die 24 is generally smaller than the memory die 22. Accordingly, the controller die 24 is conventionally placed at the top of the memory die stack. However, where there is a plurality of stacked memory die already bonded to the substrate, it is often difficult to find space on the substrate for all of the required controller die wire bonds. Moreover, there is a desire to increase the speed with which semiconductor devices operate, even as the number of memory die within a semiconductor device increases. Given these factors, some semiconductor packages are manufactured with the controller die bonded directly to the substrate.
In order to thereafter bond the memory die on top of the controller, the bottom memory die is provided with a layer of liquid adhesive. The bottom die is applied on top of the controller die so that the controller die and wire bonds embed within the layer of liquid adhesive. Thereafter, the liquid adhesive layer is cured.
This operation has certain drawbacks. For example, the liquid adhesive tends to bleed to an upper surface of the bottom memory die, where it can contaminate the die bond pads and prevent proper wire bonding to the substrate. Moreover, typically additional memory die are added on top of the bottom die to form a stack of memory die. Another problem with conventional designs is that the bottom die (with liquid adhesive) is attached at a first station, and then the package is moved to a second station, where the remaining die in the stack are mounted (using a die attach adhesive). Further problems include movement of the bottom memory die before liquid adhesive is cured, damage to the controller die or wire bonds upon embedding within the liquid adhesive, and damage to the bottom memory die upon cure and thermal mismatch between the liquid adhesive and bottom memory die.